Herzog joined Shas chairman Arye Deri, who was revealed overnight to be the subject of an investigation whose circumstances are still under a gag order. The police probe of Herzog has been going on for months but was revealed on Wednesday.

The allegations about Herzog focus on the November 2013 Labor leadership primary, in which he defeated incumbent party chairwoman Shelly Yacimovich. Police are checking whether Herzog used nonprofit organizations to fund campaign spending that went unreported.

Politicians and activists have been questioned by investigators, who are looking into whether companies spent large sums to try to help Herzog defeat Yacimovich.